This invention pertains to a process for preparing a mounting tape with metallic conductive tabs having, on at least one of their ends, metallic protrusions that can be thermowelded or soldered.
Mounting tapes are needed for automatic mounting of electronic components, a process called Tape Automated Bonding (TAB). These mounting tapes have metallic conductive tabs, the ends of which are bonded during the mounting process with the corresponding connection points on the electronic components. The bonding of the metallic conductive tabs with the electronic components is accomplished, preferably, through metallic protrusions that can be thermowelded or soldered, the so-called bumps. These consist preferably of metals softer than the conductive tabs and can be located either on the electronic components or on the conductive tabs of the mounting tape. The latter embodiment is preferred because it dispenses with the expensive process of placing metallic bumps on many different electronic components, and it permits using just one mounting tape for all components.
Known mounting tapes are monolayer tapes consisting only of a metal layer, and multilayer tapes with a dielectric support layer in addition to a metal layer with, optionally, one or more adhesive layers between the metal layer and the support layer. Mounting tapes are also known with metal layers and optional adhesive layers on both sides on the support layer. Two-layer or three-layer tapes consisting of a support layer and a metal layer, or these layers with an additional adhesive layer, are preferred. Multilayer tapes have personality windows in the support layer, and optionally in the adhesive layer, into which the metallic conductive tabs fit, thereby allowing the tabs to be easily brought into contact with the electronic components. The support layer also contains the usual sprocket holes for transport. A fundamental advantage of multilayer tapes compared to monolayer tapes is that components placed in contact with the former can be tested before final mounting, because there are no conductive connections between individual metallic conductive tabs on multilayer tapes.
Significant problems have always occurred in preparing mounting tapes with metallic conductive tabs having metallic bumps on at least one of their ends for easier and better contact with electronic components. Many preparation processes have indeed been proposed, but none has been completely satisfactory. U.S. Pat. No. 4,396,457 discloses a process for producing metallic bumps by stamping. However, these bumps are merely bulges in the conductive tabs and do not contribute to stabilizing the bonding to electronic components. In addition, this process damages, thus rendering useless, the delicate metal layer of the conductive tab by the high mechanical stress in stamping.
Other methods disclose preparing metallic conductive tabs and metallic bumps by etching processes. U.S. Pat. No. 4,411,719 and DE-A 29 26 200 (U.S. Pat. No. 4,209,355) describe processes for etching different sides of the mounting tape. In U.S. Pat. No. 3,781,596, both etching steps are performed from the same side. In EP-A 02 32 108, (U.S. Pat. No. 4,701,363) two different photoresists are applied one atop the other, such that selective deep etching of the metal can result, with simultaneous formation of conductive tabs and bumps. The disadvantage of these processes is that different resists and coating processes are required and that the exact registration of different exposure masks can be accomplished only with great difficulty on different sides of the mounting tape.
EP-A 01 17 348 (U.S. Pat. No. 4,587,395) discloses laser treatment to melt the ends of the metallic conductive tabs to metallic beads. However, all of the cited processes have the disadvantage that the metallic bumps can be prepared only from the same material as the conductive tabs. EP-A 00 70 691 and "Electronic Production", November 1988, pages 9-10, describe a mounting tape on which the metallic bumps consist of a metal, preferably gold, softer than the conductive tabs. In this process, the conductive tabs are prepared first. As gold is used as the etching resist, the process becomes extremely expensive. With the use of a second resist, gold bumps are then electrodeposited on these gold-coated conductive tabs. However, good adhesion of these bumps on the gold-coated conductive tabs can be achieved only by special pretreatment of the gold coating.
The process in U.S. Pat. No. 4,510,017 does not start with a dielectric support layer in which personality windows and sprocket holes usually have already been produced, but rather prepares dielectric sites on the mounting tape by filling the spaces between conductive tabs with dielectric material. This process also uses a gold coating as an etching resist, which makes the preparation of mounting tapes by this expensive process even more costly.
In U.S. Pat. No. 4,259,436 and EP-A 02 49 834 (U.S. Pat. No. 4,795,694), conductive tabs with bumps of metals other than those of the tabs were prepared by a combination of etching one side and plating the other side of the mounting tape. This requires two exposure masks, which must be positioned for accurate registration on different sides of the mounting tape.
EP-B 00 61 863 (U.S. Pat. No. 4,494,688) describes a process in which the metallic bumps are prepared separately on an intermediate support and then transferred onto the conductive tabs. This requires an additional support layer. In addition, transfer of the bumps is made more difficult by problems in accurate positioning and in separating the bumps from the intermediate support. During the transfer, the delicate conductive tabs can be easily bent or even destroyed.
As evidenced by the multiplicity of proposed processes, there is great interest in obtaining practical and versatile processes for preparing mounting tapes with metallic conductive tabs bearing on at least one of their ends bumps that can be thermowelded or soldered. However, the known processes involve considerable difficulties. They are protracted and expensive, require complicated process steps and exacting conditions, or they are useful only for special mounting tapes, such as monolayer tapes or those with metallic bumps of the same metal as the conductive tabs.
Therefore, the problem involved in the present invention is to make available a process for preparing mounting tapes with metallic conductive tabs bearing metallic bumps on at least one of their ends, without the disadvantages of the currently known processes. In addition, the metallic bumps should have well defined areas and heights and outstanding adhesion to the conductive tabs. The process should also produce mounting tapes with uniformly high quality and yield. Furthermore, the process should be capable of producing all types of mounting tapes and the material of the metallic bumps should not be limited to that of the conductive tabs.